This invention relates to water jet propulsion systems for marine vehicles and in particular to the pump and inlet design of water jet propulsors for planing craft.
Conventional planing craft waterjet propulsion systems consist of longitudinally installed engines and inboard mounted pumps. Nozzles and pump housings usually extend through transom cutouts, and inlet contours are faired to the hull at bottom cutouts. Matching of proper engine and pump shaft speeds is accomplished by use of step-up or step-down gearboxes where required, and various impeller sizes (usually termed "trims") are provided to match engine and pump torque characteristics. Nozzle sizes are often varied in combination with impeller trim and craft speed requirements.
For planing hull waterjet systems propulsive efficiencies at low to moderate speeds are generally much lower than for high speeds. The reason for this is that peak efficiency occurs at a fixed value of jet velocity ratio, which is the ratio of flow velocity from the nozzle to the forward boat speed, while typical planing hulls require increasing jet velocity ratios at low speeds in order to overcome hull drag. To maintain high efficiencies at low speeds some means of providing additional water flow through the propulsion unit is needed. One means of accomplishing this is by fitting a variable area nozzle to the pump, but this in turn requires variable geometry pump blading and some means of varying inlet system area as well if significant flow rate increases are to be provided. For these reasons variable nozzle systems have not been placed in general use. Since waterjet systems are not efficient at low speeds, sustained operation at these speeds is wasteful of fuel and may, for many applications, discourage the use of waterjet propulsors. When operating in the acceleration mode from stop to high speeds this inefficient region may create difficulties in terms of ability to provide adequate propulsion thrust. Failure to provide sufficient thrust in the low speed region will prevent the attainment of higher speeds where operation of the propulsion system may be satisfactory. The tendency for waterjet pumps to cavitate at low speed, high power conditions aggravates operating difficulties for conventional systems. Because pressure of the inflowing water is low while traveling at low forward boat speeds, cavitation of highly loaded pump impellers can occur under these conditions when application of full engine power is attempted. This limits the acceleration capability of the boat, and an improperly designed system can prevent attainment of normal operating boat speeds. Low operating efficiencies cause the cavitation limit to occur at higher boat speeds, thus presenting even tighter operational restrictions.
Conventional waterjet systems use engines mounted longitudinally and propulsion units mounted longitudinally with the nozzle mounted outboard of the transom. This arrangement places engines far forward of the transom, intruding on otherwise usable internal space. The pump and inlet are mounted inboard, using space and requiring the forward engine location. Installation is inconvenient and time consuming since mating with the hull inlet cutout (sometimes requiring hand-formed contours at the hull transition) and a transom cutout for the pump, nozzle and steering/reversing system is required. Maintenance and repair are likewise made more difficult by the relatively complicated installation design.
Steering of conventional waterjet systems is unresponsive at low speeds because high speed response characteristics would be oversensitive with a low speed optimized steering system. In addition, thrust at low engine speeds is typically low for waterjet systems (when compared to propellers) due to their small characteristic water flow rates. This contributes to a lack of steering response in conventional waterjet systems at low speeds.
The inboard mounting of conventional pump units requires use of positive shaft seals or packing glands which are often troublesome maintenance items. Thrust bearings capable of carrying high loads are also needed, these often being subject to failure caused by improper design, maintenance, or by contamination.